The Admiralty instructed King to discover whether there was any river 'likely to lead to an interior navigation into this great continent'. The Colonial Office had given instructions to collect information about topography, fauna, timber, minerals, climate, and the natives and the prospects of developing trade with them. From February 1818 until June the coast was surveyed as far as Van Diemen's Gulf and there were many meetings with Aboriginals and Malay proas. In June the Mermaid visited Timor and then returned to Sydney, arriving on 29 July. Next December and January King surveyed the recently discovered Macquarie Harbour in Van Diemen's Land and sailed in May 1819 for Torres Strait. King took John Oxley as far as the Hastings River, and continued on to survey the coast between Cape Wessel and Admiralty Gulf. King returned to Sydney on 12 January 1820.

Voyages of King

King's fourth voyage was undertaken in the 154 tonne sloopHMS Bathurst. The ship headed north, through Torres Strait and to the north-west coast of the continent. Further survey of the west coast was made after a visit to Mauritius.
Valuable contributions had been made to the exploration of Australia.

King had been promoted to commander in July 1821, and in April 1823 returned to England. He subsequently commanded the survey vessel HMS Adventure, and in company with HMS Beagle, spent five years surveying the complex coasts around the Strait of Magellan. The result was presented at a meeting of the Royal Geographical Society in 1831. King owned a property at Dunheved in the western suburbs of Sydney where he entertained Charles Darwin on Darwin's last night in Sydney in January 1836.